21 · Introducing OSPFOSPF 簡介

Building a Link-State Database建立鏈路狀態資料庫

When two routers discover each other and establish adjacency by using hello packets, they use the exchange protocol to exchange information about the LSAs.

當兩台路由器透過 hello 封包互相發現並建立鄰接關係後，它們會使用交換協定來交換有關 LSA 的資訊。

OSPF uses five types of routing protocol packets from which four types of OSPF packets are involved in building the LSDB.

OSPF 使用五種路由協定封包類型，其中有四種 OSPF 封包類型與建立 LSDB 有關。

Type
類型
Packet Name
封包名稱
Description
說明
1
1
Hello
Hello
The hello packet discovers and maintains neighbors.
hello 封包用於發現並維護鄰居。
2
2
DBD
DBD
The DBD packets describe the summary of the LSDB and contain the LSA headers that help routers build the LSDB.
DBD 封包描述 LSDB 的摘要，並包含協助路由器建立 LSDB 的 LSA 標頭。
3
3
LSR
LSR
After DBD packets are exchanged, each router checks the LSA headers against its own database. If it does not have current information for any LSA, it generates an LSR packet and sends it to its neighbor to request updated LSAs.
交換 DBD 封包後，每台路由器會將 LSA 標頭與自己的資料庫進行核對。如果某個 LSA 沒有最新的資訊，路由器就會產生一個 LSR 封包，傳送給鄰居以要求更新的 LSA。
4
4
LSU
LSU
The LSU packets contain the requested LSAs that should be updated. This packet is often used in flooding.
LSU 封包包含要求更新的 LSA。此封包也常用於泛洪。
5
5
LSAck
LSAck
LSAck packets help to ensure the reliable transmission of LSAs.
LSAck 封包有助於確保 LSA 傳輸的可靠性。

As shown in the table, the exchange protocol operates as follows:

如表格所示，交換協定的運作方式如下：

  1. The routers exchange one or more database description (DBD) packets. A DBD includes information about the link-state advertisement (LSA) entry header that appears in the LSDB of the router. Each LSA entry header includes information about the link-state type, the address of the advertising router, the cost of the link, and the sequence number. The router uses the sequence number to determine the "newness" of the received link-state information.路由器會交換一個或多個資料庫描述（DBD）封包。DBD 包含有關該路由器 LSDB 中鏈路狀態通告（LSA）項目標頭的資訊。每個 LSA 項目標頭都包含鏈路狀態類型、通告路由器的位址、鏈路成本，以及序號等資訊。路由器會使用序號來判斷所收到鏈路狀態資訊的「新舊程度」。
  2. When the router receives the DBD, it acknowledges the receipt of that DBD by responding with its own DBD.當路由器收到 DBD 時，會以回傳自己的 DBD 來確認已收到該 DBD。
  3. The routers compare the information that they receive with the information that they have. If the received DBD has a more up-to-date link-state entry, the router sends an LSR to the other router to request the updated link-state entry.路由器會將收到的資訊與自己擁有的資訊進行比較。如果收到的 DBD 中有更新的鏈路狀態項目，路由器就會向另一台路由器傳送 LSR，要求更新的鏈路狀態項目。
  4. The other router responds with complete information about the requested entry in an LSU packet.另一台路由器會以 LSU 封包回應，其中包含所要求項目的完整資訊。
  5. When the router receives an link-state update (LSU), it adds the new link-state entries to its LSDB and it sends an LSAck.當路由器收到鏈路狀態更新（LSU）時，會將新的鏈路狀態項目加入其 LSDB，並傳送 LSAck。
Which packet is used to ensure a reliable transport and a reliable exchange of information?使用哪一種封包來確保可靠的傳輸與可靠的資訊交換？